The present invention relates to interlocking roof tiles which are made of a cementitious mixture such as concrete and formed by extrusion and more particularly but not exclusively to interlocking roof tiles which are to be laid in broken bond. In this specification the term roof tiles also includes “cladding tiles”.
Interlocking roof tiles have a generally rectangular configuration when considered in plan and have an upper and under surface (in use), two oppositely facing side edges, an upper edge and lower (leading) edge which is visible in use when the tile is laid in overlapping relationship with at least one tile of a next adjacent row of tiles, an underlock extending along one of the side edges and an overlock extending along the other of the side edges. The over and underlocks respectively engage with the over and underlocks of adjacent tiles of the same row.
A major advantage of interlocking tiles over both natural slate and concrete plain tiles is that the necessity to double lap the tiles is removed thereby reducing the number of laid tiles per roof and consequently the cost. With natural slate and concrete plain tiles, the thickness of the visible lower edge when the tiles are laid (commonly 10 mm and 12 mm, respectively) provides an aesthetically pleasing appearance. Therefore a further objective of interlocking tiles is to retain the aesthetic appearance produced by broken bond laying, and maintain a visible lower edge similar to existing natural slate and concrete plain tiles, in particular in the case of retiling old roofs.
Early interlocking concrete slate tiles had a substantially flat geometry with relatively thick lower edges (25 mm) when compared with natural slate and concrete plain tiles. This thickness was thought necessary to provide sufficient strength to avoid “breaking off” and to accommodate the interlocks (i.e. the over and underlocks extending along opposite side edges respectively of each tile). The thickness and resulting mass of these tiles deemed them an unsuitable replacement for natural slate.
An interlocking tile with a thin lower edge was achieved (DE 93888) but these tiles were inherently incapable of being extruded by virtue of their two layered design and had to be made by moulding.
In general, moulded or pressed tiles provide a larger range of shape and dimension options than extruded tiles. However, as the tile production rate by moulding is low and therefore more costly, extrusion is preferred as a manufacturing method.
Interlocking roof tile production by extrusion of cementitious materials including cement, sand, fillers and pigment, involves apparatus including an extrusion head comprising a hopper-like box which is disposed above a conveyor path and which is charged with the cementitious mixture. The flow of the cementitious mixture is assisted in the box by means of a rotating paddle. A succession of pallets for moulding the under surface of the tiles is driven along the conveyor path and past the box so that the cementitious mixture from the box forms on the pallets and is compressed thereon by means of a rotating roller mounted within the box downstream of the paddle, and having a contour which corresponds to the upper surface of the tiles to be formed.
The cementitious mixture is further compressed on the pallets as they pass out of the box by means of a slipper which is disposed downstream of the roller and also has a contour which corresponds to that of the upper surface of the tile to form a continuous extruded ribbon of cementitious mixture on the pallets.
A limiting feature of the upper surface contour being formed in this way is that only features of shape which lie in the direction of extrusion can be formed because any features transverse to this direction will be removed by the action of the roller and slipper.
The extruded ribbon of cementitious mixture on the pallets is subsequently cut into tile forming lengths downstream of the box by means of a suitable cutting knife, optionally with nail holes being formed in the tile forming lengths at the same time. The pallets with the formed tiles thereon are then conveyed to a curing chamber where the tiles undergo only a partial curing and are then conveyed to, and stacked, out-of-doors to complete the curing process.
After partial curing, the tiles are depalleted, for example by means of rotating depalleting wheels disposed on opposite sides of the conveyor path. In operation, the wheels successively enter in between the pallets and tiles, with the tiles continuing along the original conveyor path, and the pallets being carried downwards along a different conveyor path, thereby separating the tiles from the pallets.
The applicant has previously manufactured interlocking extruded concrete slate tiles with a thin leading edge (approximately 10 mm), as described in EP 0387305. This tile has the generally flat geometry of natural slate and a hidden interlock. The thin leading edge is achieved by the under surface portion adjacent to the leading edge being inclined at an angle with respect to the upper surface (i.e. thinned). The interlock is hidden from view by virtue of a cut-out at the leading edge region of the underlock which cut-out is occupied when the tile is laid by an extending portion of the leading edge on the opposite side edge. This not only shields the interlock but it also provides a leading edge line without any ‘rat holes’.
Rat holes form between the underlock of each tile in one row and the upper surface of each overlapping tile in the row below if interlocking tiles with underlocks which extend the entire length of one of the side edges are laid together to form a roof. This is a consequence of the under surface of the underlock being raised in relation to the lower line of the leading edge so that the two would appear staggered when viewed from the tile leading edge. Rat holes are undesirable as they are both unsightly and allow the ingress of wind driven rain into the roof space.
It is a bonus that in a laid roof of the interlocking tiles described in EP 0387305, the hidden interlocks between adjacent tiles, provide a straight leading edge line that has an aesthetically pleasing appearance. The tile can also be manufactured with a longitudinal groove running the full length of the tile thereby forming a one-piece two tile plain tile.
The applicant has also manufactured an extruded interlocking roof tile, as described in GB 2327954, also with a hidden interlock and being essentially flat, i.e. having the same overall thickness at the upper and leading edges. The upper and lower edges of this flat interlocking roof tile are presented by respective transverse ridges. These transverse ridges are of substantially equal overall thickness and present the lowest point of the tile with the tile under surface having regions that extend along the two side edges and which are substantially mutually parallel. This flat interlocking tile is provided with a hidden interlock to which end the transverse ridges are coterminous with a laterally extending surface of the overlock, and the underlock terminates short of the lower edge to provide a cut-out, such that when the tile is laid in overlapping relationship with an adjacent similar tile, a ridge portion adjacent the overlock is received in the cut-out whereby to shield the interlock from view.
In forming the cut-outs mentioned above, the tile forming material being removed (a so-called coupon) to form the cut-out must be cleanly and quickly achieved and subsequently removed without damaging the tile. To achieve this, extensions to the cutting knife or other modifications to existing equipment used in the extrusion process for making such tiles are known but involve some considerable expense and inconvenience over and above the existing extrusion process. Care must be taken in facilitating outward displacement and avoiding risk of damaging the tile during outward displacement of the coupon by the cutting knife.
An example of one such cut-out formation method, in the manufacture of flat interlocking roof tiles, is described in GB 2327954. The apparatus for forming the coupon in GB 2327954 includes a resilient cutting blade appended to the cutting knife used for severing the extruded ribbon into tiles. Coupon displacement from the main body of the ribbon is achieved by the resilient cutting blade co-operating with an inclined surface upstanding from the pallet surface. The inclined surface is presented by a ridge portion which forms an L-shaped ridge and which forms an L-shaped depression in the undersurface of the ribbon. By means of the resilient cutting blade, a line of separation extending substantially parallel to the direction of extrusion is formed in the ribbon and when the resilient cutting blade engages the inclined surface on the pallet, it flexes outwardly thereby exerting an outwardly directed force on the tile forming material to displace the coupon of tile forming material outwardly of the tile.
Unless barriers are provided in front of the cut-outs in such tiles, wind driven rain may ingress into the roof space. Such barriers are provided in both the interlocking roof tiles mentioned above, and described in EP 0387305 and GB 2327954, by transverse ridges at the leading edge on the underside of the adjacent tile which occupies the cut-out. When two tiles are laid in side-by-side interlocking relationship, the underlock and overlooks matingly engage and the cut-out is filled by an extending portion of the leading edge on the opposite side edge. A portion of the transverse ridge then provides a barrier which both shields the interlock from view and prevents wind driven rain entering the roof space.
Such barriers as described in the preceding paragraph, may also be necessary in contoured tiles, in particular contoured tiles to be laid at low pitch. Contoured tiles, as described in GB 1174992, have a trough (pan) which extends between the upper and lower edges of the tile. In order to prevent the passage of water between adjacent surfaces of two vertically adjacent contour tiles, the tile is provided with a barrier means extending across the trough and inset from the leading edge of the tile. The barrier means cannot be formed on the tile as part of the normal extrusion process as it is transverse to the direction of extrusion and would be removed by the action of the roller and slipper. Therefore, the barrier means on the contour tile is formed by pressing a bar of material, from which the tile itself is being made, into the material of the ribbon before the tile forming length is severed from the ribbon, as described in GB 1174993. Again the disadvantages associated with such a method include the additional equipment and processing steps required in addition to the basic tile forming extrusion procedure. Associated with this are additional costs and increased risks of interruptions in the manufacturing process and consequential undesirable down time.